Rename the entity_ref module to 'entity'.

This is in preparation for moving a number of data structures into
sub-modules of the 'entity' module:

- EntityList
- EntityMap
- SparseMap

lib/cretonne/src/regalloc/diversion.rs

@@ -104,7 +104,7 @@ impl RegDiversions {
 mod tests {
     use super::*;
     use ir::Value;
-    use entity_ref::EntityRef;
+    use entity::EntityRef;
 
     #[test]
     fn inserts() {

lib/cretonne/src/regalloc/liverange.rs

@@ -435,7 +435,7 @@ impl SparseMapValue<Value> for LiveRange {
 mod tests {
     use super::LiveRange;
     use ir::{Inst, Ebb, Value};
-    use entity_ref::EntityRef;
+    use entity::EntityRef;
     use ir::{ProgramOrder, ExpandedProgramPoint};
     use std::cmp::Ordering;
 

lib/cretonne/src/regalloc/solver.rs

@@ -702,7 +702,7 @@ impl Solver {
 #[cfg(test)]
 #[cfg(build_arm32)]
 mod tests {
-    use entity_ref::EntityRef;
+    use entity::EntityRef;
     use ir::Value;
     use isa::{TargetIsa, RegClass, RegUnit};
     use regalloc::AllocatableSet;